Removal of chromium from tannery industry wastewater using iron-based electrocoagulation process: experimental; kinetics; isotherm and economical studies

Chromium is a hazardous compound from industrial processes, known for its toxicity, mutagenicity, teratogenicity, and carcinogenicity. Chemical methods are efficient but cost-effective alternatives with reduced sludge are sought. Electro-coagulation, utilizing low-cost iron plate electrodes, was explored for factual tannery wastewater treatment in this manuscript. Operating parameters such as initial chromium concentration, voltage, electrode number, operating time, agitation speed and current density has been studied to evaluate the treatment effeciency. Under optimal conditions (15 V, 0.4 mA/cm2, 200 rpm, 330 ppm chromium, 8 iron electrodes with a total surface area of 0.1188 m2, 3 h), chromium elimination was 98.76%. Iron anode consumption, power use, and operating cost were 0.99 gm/L, 0.0143 kW-h/L, and 160 EGP/kg of chromium eliminated, respectively. Kinetics studies were pursued first-order reaction (97.99% correlation), and Langmuir isotherms exhibited strong conformity (Langmuir R2: 99.99%). A predictive correlation for chromium elimination (R2: 97.97%) was developed via statistical regression. At HARBY TANNERY factory in Egypt, industrial sewage treatment achieved a final chromium disposal rate of 98.8% under optimized conditions.


Materials
The reagents utilized in this experiment were potassium chromate of 98% purity, hydrochloric acid (HCl) of 30% purity, sodium hydroxide Pellets (NaOH) with a purity of 98% and sodium chloride (NaCl) of 98% purity.All reagents were obtained from El-Gomhouria Company for Trading Chemicals and Medical Appliances located at greater Cairo in Egypt.

Samples collection and classification
Factual tannery wastewater samples are assembled from a leather tanning factory in the industrial area of Rubiki (east of Cairo).These samples are transported and stored at 4 0 C to be analyzed per APHA 2017 standards 1 .To assess disposal efficiency under varying conditions, we created a chromium stock solution by dissolving 98% pure potassium chromate salt in distilled water.We achieved the desired experimental concentrations through successive dilutions with distilled water.Table 1 reveals the factual tannery wastewater characterizations.

Electro-coagulation system
The experimental unit as shown in Fig. 1 includes a cylindrical glass container (reactor) with internal diameter of 10.5 cm and total a capacity about 1.2 L. This design allows setting 10 equidistant iron electrodes.Each iron electrode has a rectangular Sect.(13.5 cm × 5.5 cm) with thickness of 1 mm.Electrodes are vertically positioned, and arranged parallel to each other with a space of 1cm between them and installed at 2 cm from the bottom of the reactor.The electrodes operate in monopolar mode and connect to the positive and negative depots of the DC power supply (Range 230 V/50 A).The magnetic stirrer is utilized to maintain an unchanged composition and avoid the association of the clumps in the solution.
The anode efficient area (EA) can be calculated as shown in Eq. (1) 33 .The immersed length here will be 9 cm form plate height (13.5 cm).

Experimental proceeding
In each trial, 500 mL of the sample was placed in the glass reactor.Optimum pH for chromium elimination is preferred within the range 4-8, as suggested by previous studies 34,35 .The observed pH increase is owin to the generation of hydroxide ions (OH -) from the cathode through water electrolysis, as described in Eq. (3), which is used to produce hydroxides or bind to the sludge 36 .Elevated pH values, primarily Fe(OH) 3 , positively impact contaminant removal, consistent with many litrature.OH -ions can also undergo partial combination with Cr 3+ ions, resulting in the formation of the insoluble hydroxide precipitate Cr (OH) 3 .Interestingly, when the initial pH was raised to 9.00, the efficiency of total Cr removal decreased because of the conversion of Cr ions to CrO 4 2-, which in turn weakened the effectiveness of Cr disposal 37 .The solution pH is adjusted by adding HCl (10%) and/ or NaOH (10%) solutions as required.A specific concentration of NaCl (98% purity) is added for supporting electrolyte and depending on the experiment to increase the conductivity.All experiments were carried out at room temperature (25 0 C).At specified time intervals; 30 mL supernatant was withdrawn by a pipette from the top of the reactor for chromium measurement.The final concentration of chromium in the solution was determined by UV-VIS spectrophotometer; model UV-BK1900 with a 1 cm quartz cell at 540 nm.The produced sludge was analyzed by XRF analyzer.Table 2 indicates the removal efficiency combined to the change in a lot of factors to investigate the reaction optimum conditions.The chromium removal percent was appraised using next equation: where C 0 and C t are the initial and the final value of chromium, (mg/L).

Chromium elimination mechanism
Elimination process of chromium is done according to the oxidation and reduction reaction at anode and cathode respectively according to the next equations

Economical study
In this manuscript, plate material and electrical power price were considered as main price terms in the processing costs estimation (EGP/gm chromium eliminated) utilizing the succeeding equation electrodes consumption can be calculated from the next Eq. 38,39.
Power exhaustion, E is estimated from the below equation at the optimum processing factors 40,41 .
where a is the electrical power price, (EGP/kWh) and it taken as 1 kWh = 1.60 EGP, b is the plate material price, (EGP/t iron ) the price 1 t of iron = 27,500 EGP.

Ethics approval
Not applicable.This manuscript does not involve researching about humans or animals.

Consent to participate
All of the authors consented to participate in the drafting of this manuscript.

Influence of treatment time on chromium concentration
The required treatment time has a significant effect not only in effluent quality but also in the treatment cost and reactor volume 42 .The impact of reaction time on chromium elimination efficacy was inspected under several process time values ranging from 1 to 5 h, while other operating factors were kept at constant values; i.e. 15 V, 0.4 mA/cm 2 current density, 330 ppm initial chromium concentration, 200 rpm, and 6 Fe electrodes.It is obvious as shown in Fig. 2 that the elimination efficacy rises with the increase in treatment time.3 h were enough to achieve removal efficiency of 97.01%which also almost equal to maximum the removal efficiency after 4 h 97.25%.Economically, time reaction 3 hours should be considered.The elimination efficacy reduces with increasing time which may be because of the influence of the electro-coagulation attaining the saturation point.Thus, unrestricted progress is not accomplished by raising the reaction time.As time increases, metal sheets develop a thin protective passivation film, affecting the quantity of deteriorated Fe(II) plates, generating radicals, reducing ions, and decreasing flocculant levels, while also diminishing the oxidation effect.Another crucial factor to consider in electro-coagulation is power consumption, influenced by longer operation times, leading to higher treatment costs.This is mainly due to the rapid oxidation of Cr 3+ and the formation of Cr(OH) 3 .Initially, fine particles tend to aggregate during electro-coagulation, but as treatment time extends, particle size increases, resulting in reduced chromium disposal efficiency 43 .One possible explanation is that after 8 h, deposited chromium returns to the liquid phase, leading to decreased chromium disposal efficiency and an increased ratio of total dissolved solids.

Influence of initial chromium concentration on treatment efficiency
Figure 3 indicates that the disposal efficiency reduces with a raise in an initial chromium concentration.This performance is a result of the lack of clumps for a sorption of extra chromium at elevated concentrations and likewise because of the minimum iron corrosion rates and rising iron surface passivation at elevated chromate concentrations 44,45 .The cause why the chromium elimination efficacy reduces with raising in its premier concentration is concluded from Faraday's law.In line with Faraday's law, a fixed current density yields a steady release of Fe 2+ into the solution through anodic electro-dissolution, which intensifies with prolonged electrolysis 46,47 .These ions play a a pivotal role in facilitating the reduction of Cr(VI) to Cr(III), leading to the formation of insoluble Cr(OH) 3 and Fe(OH) 3 .Subsequently, when the initial chromium concentration rises, a substantial quantity of iron ions becomes essential during prolonged electrolysis periods to achieve complete chromium ion reduction 29,48 .Alternatively, one can increase either the current density or the electrode surface area proportionally to ensure sufficient Fe 2+ production for effective Cr(VI) disposal from the effluent 49 .

Influence of applied voltage and current density on treatment efficiency
The applied voltage is a crucial parameter that plays a critical role in the electro-coagulation technique.The data depicted in Fig. 4 exhibits that an increase in applied voltage corresponds to a greater degree of chromium disposal.The maximum elimination percentage was 97.01% at voltage of 15 V then there was a decrease in the elimination percentage with additional voltage.For that reason, the applied voltage was elected 15 V for chromium elimination during the electro-coagulation technique.Hasan et al. 42 reported the similar observation and achieved utmost chromium efficacy at 15 V. Increasing current density enhances metal removal efficiency by boosting Fe 2+ and OH -production 40,50 .Current density impacts electro-chemical metal dosing rate, electrolytic bubble production, and floc growth,while also influencing electrical energy consumption, electrode material usage, and overall operating costs in electrocoagulation 51,52 .The same results were in the current density as shown in Fig. 5.The chromium elimination percentage is raised with raising the current density.In accordance with Faraday's law, this observation was respected owing to an anodic dissolution 28,39,45 .In contrast, when a current density of 0.54 mA/cm 2 is applied, a slight reduction in the elimination percentage is noticed.This reduction can be attributed to the formation of a higher amount of hydrogen bubbles at the cathode, which causes the sludge to rise and hinders the formation www.nature.com/scientificreports/ of flocs 28,53,54 .As a result, the chromium removal percent is reduced.It is noticed that with raising the current density, the turbidity of wastewater was also raised.As per Moussa et al. 's findings in (2017) 55 , this behavior can be ascribed to a critical current density threshold.Even when using higher current density, the treated water does not exhibit a significant improvement beyond this critical point.Elevated current densities result in the release of a notable quantity of Fe 3+ ions through anode dissolution.As a result, the generated Fe(OH) 3 molecules adhere to surfaces without the presence of contaminants due to the saturation of adsorption sites created by iron hydroxide 56 .The utilization of high working current values has been linked to the generation of residual energy, leading to an increase in water temperature 7 .For that reason, 0.4 mA/cm 2 of current density believed the suitable current density for the technique electro-coagulation.
The impact of current desity on the power consumption was also elucidated under several values ranging from 0.15 to 0.54 mA/cm 2 while other operating factors were kept at constant values; i.e. 15 V, 330 ppm initial chromium concentration, 200 rpm, and 8 Fe electrodes for 3 h.It is obvious as depicted in Fig. 6 that the power consumption is directly related to the current density.Increasing current density from 0.15 to 0.4 mA/ cm 2 boosted chromium disposal efficacy from 89.84% to 98.76%, with power consumption rising from 0.0053 to 0.0143 kW-h/L.In a separate study, raising current density from 0.42 to 0.94 mA/cm 2 improved chromium removal efficiency from 74.35% to 100%, alongside an energy consumption increase from 0.24 to 0.94 kW-h/m 3 .

Effect of magnetic rotational speed and number of fe electrodes on treatment efficiency
Rotational speed is very important as it ensures that the flocculants from the dissolved electrodes are homogeneously dispersed in the reactor.Referring to Fig. 7, the suitable magnet agitation velocity can be 200 rpm.In the chromium elimination process, iron and hydroxyl ions combine to create highly absorbent iron hydroxide that binds to contaminants.Iron hydroxide also forms aggregates with a network structure, effectively removing contaminants from the liquid.Excessive agitation can disrupt these aggregates, releasing contaminants 57 .Also, high agitation speeds lead to increased shear rates at the floc interface, resulting in irreversible floc breakage and preventing re-growth 58 .www.nature.com/scientificreports/ The effects of rising electrode number on the elimination efficacy are elucidated in Fig. 8. From the figure, it was noticed that the disposal efficiency was raised with raising number of Fe electrodes from 4 to 8 electrodes.This elimination is owing to the destabilization of Cr in the solution through the hydrolysis products of iron, which permits accumulation and a superior segregation of the solution thru sedimentation or flotation.
It was noticed that the simultaneous elevate in the plates number improves disposal efficacy.This could be owing to the existence of ferrous hydroxides, which raises with the plates number.These composites perform as coagulants and trap contaminant molecules, perhaps thru the relative rise in Fe(II) hydroxides beside the plates number 59 .The outcomes also indicate that elevating the effective area enhanced the disposal efficiency and reduced processing time beloved, the elucidation of this observation as obeys: greater plate surface area led to a greater dispersion of bubbles everywhere the reactor, whilst a minimal plate surface area led to a massive  www.nature.com/scientificreports/bubbles source inside the reactor, and as the bubbles dispersion inside the reactor, the likelihood of collision between the bubbles and coagulant rises causing to rise the elimination efficacy.
The outcomes also indicated that the elimination efficiency relatively constant in case of 10 electrodes applied.For that reason, the ideal number of electrodes was considered 8 electrodes for chromium elimination through the electro-coagulation process.

Kinetic investigation via treatment process
The kinetic models of Cr elimination from wastewater were fulfilled under the ideal processing factors (330 ppm premier chromium concentration, 0.4 mA/cm 2 current density, elimination time of 3 h, 8 iron electrodes, 200 rpm, and applied voltage of 15 V).Adequate results of Cr elimination kinetics are revealed in Table 3.The linear formula of first and second order kinetic models can be introduced in the following equations: where C o and C t are the premier and final chromium concentrations, respectively.K 1 and K 2 are first and second order rate constants in min −1 and L. gm −1 min −1 , respectively, and t is the treatment time (in min).A plot lnC t and [ 1 Ct − 1 Co ] against time for each run leads to a straight line whose slope is K 1 and K 2 , respectively.It can be observed that the correlation coefficient R 2 of the pseudo first order kinetic model was better than that of the pseudo second order kinetic model.An identical examination has been mentioned before by 37 in the elimination of chromium from tannery wastewater by electro-coagulation.Also, Bingül et al. 60 and Lorgio Valdiviezo-Gonzales et al. 7 reported the first order kinetic model with good correlation for chromium.The regression analysis of the concentration curves against treatment time denotes that the reaction rate can be depicted by means of first order kinetic model for chromium elimination.The end result is exhibited in Fig. 9.

Isothermal modeling investigation
Isotherms are employed to depict the adsorption process under ideal factors ((330 ppm premier chromium concentration, 0.4 mA/cm 2 current density, 8 iron electrodes, 200 rpm, and applied voltage of 15 V), and the Langmuir and Freundlich isotherms are widely used for this purpose.The Langmuir isotherm specifically characterizes adsorption at uniform sites, where a monolayer is formed.It is represented by a linear equation, given by the below Eq. ( 13) 49 : The adsorption capacity q e (mg/gm) was calculated by means of the bellow Eq. ( 14):  where q e (mg/gm) is the equilibrium concentration of chromium, C e (mg/L) is the equilibrium concentration in the liquid phase, V and W are volume of solution (L) and the weight of a sorbent (gm), individually.A linear plot of C e /q e versus C e indicates that the adsorption data reasonably fits the Langmuir isotherm, as exhibited in Fig. 10.The constants were obtained from the slope (1/q m ) and intercept (1/K L ) and are listed in Table 4.The Langmuir equation, expressed in terms of the dimensionless factor R L , is expressed by: The value of R L , a positive number (0 < R L < 1), denotes the feasibility of the sorption process.The Freundlich isotherm model is based on the assumption that adsorption takes place on heterogeneous surfaces with varying adsorption energies.The linear form of the Freundlich equation is expressed as: where 1/n is the heterogeneity factor related to intensity, and K f is the Freundlich constant.The slope and intercept of log q e against log C e give 1/n and K f values as elucidated in Fig. 10b.
The comparison of correlation values (R 2 ) between the Langmuir and Freundlich adsorption isotherms indicates that the Langmuir isotherm outperforms the Freundlich isotherm.This indicates a robust correlation between the experimental data and the Langmuir model, suggesting that the Langmuir adsorption isotherm accurately describes the adsorption behavior in the studied system.The Langmuir isotherm is consistent with prior work 61 .

Sludge characteristics
The formed sludge was gathered, dried at 103 0 C for 24 h, and then cooled in desiccators to assess the surface morphology of the deposited sludge throughout the XRF analysis.The XRF results showed that the chemical composition of the sludge ranged from 70-75% as weight of iron oxide and the percent of the chromium was ranged from 25-30% as weight from the simulated wastewater and the ferric and ferrous hydroxides formed during electro-coagulation later turn into magnetite.These results were in the same line of Un et al. 30 who stated that the percent of the chromium in the produced sludge in his study was 16.6% as weight and 74.3% as weight of chromium which confirms our finding and our hypothesis of iron oxide forming.

Statistical analysis for the proposed treatment process
To elucidate the influence of the operative factors on the chromium elimination efficiency, a mathematical correlation must be recommended.Statistical and least square multivariate regression techniques are extremely utilized for modeling and analysis of troubles in which a response of interested dependent variable is influenced ( 14)  www.nature.com/scientificreports/by numerous independent variables.This model was utilized in an attempt to detect the mathematical correlation that can clarify the influence of the operative factors on the chromium disposal efficiency (ANOVA).The values and p-values of the coefficients are provided in Table 5.The p-values decides if any given factor is momentous or not.The correlation terms having a p-value less than 0.0001 are momentous.The R 2 was 97.97%; it exhibited that the changeability in the adsorption could be explicated by the model, with the cohesion between the experimental and predicted values being momentous inside the process.The acquired correlation in terms of momentous factors only has the following formula: where CD is the current density, C o is the chromium initial concentration, AV agitation velocity, t is the elimination time, and N is the number of iron electrodes.
The normal probability of standardized residuals with average correlation errors of zero is demonstrated in Fig. 11.The linear distribution of the residual errors elucidates that the errors are normally distributed which denotes that the model prognoses are not prejudiced and for more clarity the comparison between the experimentally observed values of chromium removal percentage and predicted values is shown in Fig. 12.The figure exhibits a perfect convention between them.Table 6 clarifies the experimental observed and predicted chromium elimination percentage for the 19 runs.The chromium elimination percentage varied from 49.53% to 98.79%.

Application of electro-coagulation on chromium disposal from factual tannery wastewater with economical analysi
Chromium removal from sewage obtained from HARBY TANNERY in Rubiki (Badr city) was studied.The initial characteristics of the tannery sewage were 2700 mg/L total suspended solids, 1750 mg/L BOD5, 4025 mg/L COD, pH (6-9), NH4 (100 mg/L), TKN (250 mg/L), oil and gas (140 mg/L), sulphide (350 mg/L), and 3300 mg/L chromium.Electro-coagulation was conducted under optimized conditions (15 V applied voltage, 0.4 mA/cm 2 current density, 200 rpm, and 330 ppm chromium initial concentration using 8 electrodes for 3 h).The real tannery wastewater concentration was reduced from 3300 to 330 ppm through dilution with distilled water.These results indicate a remarkable chromium elimination rate of 98.8% under these optimized conditions.A comparison with previous electro-coagulation studies on tannery wastewater treatment is exhibited in Table 7. (17)   Removal % = 167.24+ 13.68 CD−0.0687C 0 − 0.04987 AV−23.14N + 3.941 t−48.3636CD 2 +8.008 t * N−6.506 t 2 www.nature.com/scientificreports/Economic studies have been used for the purpose of commercialization and many studies used the economical in their research to make the research easy use [62][63][64] .Based on the Eqs.(8-10), it was found that under optimized conditions (330 ppm initial chromium concentration, 0.4 mA/cm 2 of current density, treatment time of 3 h, 8 iron electrodes, 200 rpm, and applied voltage of 15 V), the sacrificial iron anode exhaustion per liter of the effluent was 0.99 gm Fe per L. and the power exhaustion, E was 0.0143 kW-h/L of treated wastewater.The final Processing cost was (0.1546 EGP/gm (cr) removed).
A kilogram of pure chromium amounts to the equivalent of 4,500 EGP, and thus it is a profitable value for the idea of working the cell to remove and recover chromium in the future.

Conclusion
Electro-coagulation technique was used to remove chromium from wastewater of tanning and other leather processing.The recent study aimed to determine the optimum operation conditions which achieve high removal efficiency taken into account the economic concept and it concluded that: • For raw wastewater with 330 ppm of chromium, elimination efficacy of 98.76% can be achieved at the condi- tions of 15 V, 0.4 mA/cm 2 current density, 200 rpm, 8 plate of electrodes and reaction time of 3 h.• The chromium elimination followed the 1 st order reaction by kinetic analysis of electro-coagulation technique with a 97.99% correlation factor (R 2 ).www.nature.com/scientificreports/ • The chemical composition of the deposited sludge after treatment ranged from 70-75% as weight of iron oxide and the percent of the chromium was ranged from 25-30% as weight from the simulated wastewater and the ferric and ferrous hydroxides formed during electro-coagulation later turn into magnetite.• The acquired correlation, considering significant factors only, can be expressed by the following formula: • The maximum elimination percentage of chromium from sewage assembled from the effluent stream of the HARBY TANNERY factory in Rubiki (Badr city) reached 98.8% under the aforementioned optimal processing conditions.The 98.8% removal achievable exceeds most prior electro-coagulation studies, highlighting the optimization of operating conditions.• At optimized conditions, each 326 removed ppm from chromium consumes 0.99 gm/L from iron anode electrode, 0.0143 kw-h/L power and costs about 0.05 EGP.• In comparison to chemical coagulation, electro-chemical treatment is a faster and more efficient and cost- effective technique for wastewater treatment, providing a sustainable treatment alternative.It requires lower coagulant doses and shorter treatment times, making it an efficient and economical choice.In addition, the using of modeling isotherm with these combinations of economical, kinetics and using real waste may decrease the effort and cost that could be paid for doing such pilot-scale work for electrochemical treatment of tannery wastewater.

Figure 1 .
Figure 1.Schematic diagram of the used system for electro-coagulation.

Figure 9 .
Figure 9. Reaction rate of 330 ppm initial chromium concentration at ideal processing factors.a First order kinetic model.b second order kinetic model.

Figure 10 .
Figure 10.Isothermal models for the chromium adsorption onto carbonized rice straw: a Langmuir isotherm model.b Freundlich isotherm model.

Table 1 .
Characterization of factual tannery wastewater.

Table 2 .
Experimental range of test factors studied.

Table 3 .
Reaction rate of 330 ppm initial chromium concentration.

Table 4 .
Results of isotherms for chromium adsorption of onto carbonized rice straw.

Table 5 .
Value and p value of all coefficients.

Table 6 .
Experimental observed and predicted chromium elimination percentage.

Table 7 .
Comparison of current outcome with other consequences.